To improve the surface properties of a steel sheet by optimizing hot rolling conditions and heating conditions at the time of plating.
At the time of executing finish rolling, by a continuous rolling mill, hot rolling is executed in such a manner that the finishing temp. FT(°C) in the finish is regulated to <800°C and the draft in a final stand is regulated to R(%), which is cooled in such a manner that the average cooling velocity for 3sec from the completion of the rolling is regulated to v(°C/sec) and is coiled in such a manner that CT(°C) is regulated to <600°C. In this case, these four factors are required to satisfy formula I and inequality II. This conditions are set for the purpose of thinning scale on the surface of the hot rolling coil, improving the efficiency in a pickling stage and improving the yield of the material after the finish rolling. Then, the coiled steel sheet is pickled, is thereafter heated (preferably to ≥600°C) and is galvanized in a state in which unrecrystallized structure remains by ≥20%. In this way, the level of the strength of the steel sheet can be regulated to 400 to 510MPa.
KOBAYASHI SATOO
OMURA MASAKI
